1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus.
2. Description of the Related Art
In recent years, it is examined that an image forming apparatus provided with an exposure device in which a laser light source scans and exposes a photosensitive drum employs a vertical-cavity-surface-emitting semiconductor laser as the laser light source in order to raise productivity and resolution thereof. The vertical-cavity-surface-emitting semiconductor laser emits a laser beam from a wafer surface.
The vertical-cavity-surface-emitting semiconductor laser has an advantage that the number of beams can be easily increased because of a two-dimensional arrangement of emission points in comparison with the edge emitting type semiconductor laser that emits a laser beam from the wafer edge. The vertical-cavity-surface-emitting semiconductor laser has potential as a laser light source of an exposure device in view of this advantage.
On the other hand, the vertical-cavity-surface-emitting semiconductor laser has a characteristic that a rise time from a supply timing of driving current to a risen timing (a timing at which light amount reaches target light amount) of the laser light source is longer than that of the edge emitting type semiconductor laser under the constant current control.
The long rise time of the laser light source thins a latent image. When the thinned latent image is developed as-is, an image quality deteriorates.
Particularly, the electrophotographic image forming apparatus is required to shorten the rise time of a laser light source in order to raise productivity and resolution by increasing modulation frequency of the laser light source.
The vertical-cavity-surface-emitting semiconductor laser has a characteristic that a rise time varies with internal temperature or its ambient temperature. Here, a variation of the rise time of the vertical-cavity-surface-emitting semiconductor laser as the temperature change will be described with reference to FIG. 7. FIG. 7 is a graph showing rising waveforms of a video signal Vo, driving current Id′, and optical output of an emission section, when driving the emission section of a vertical-cavity-surface-emitting semiconductor laser by the conventional constant current control.
As shown in FIG. 7, when the video signal Vo on an H (High) level is inputted, the driving current Id′ starts to supply. When the driving current Id′ starts to supply, the optical output of the emission section (the waveform of the laser light) immediately rises to a certain light amount, and then, the light output gradually rises until it reaches a target light amount according to a time constant of a current control circuit. The delay between the rising of the optical output and the rising of the driving current Id (the video signal Vo) is influenced by temperature (temperature of the emission section or its ambient temperature). The rise time becomes longer in lower temperature (a solid line), and the rise time becomes shorter in higher temperature (a dotted line). Such a variation of the rise time of the emission section causes deterioration of image quality.
Therefore, the following driving system has been proposed in order to shorten the rise time of a vertical-cavity-surface-emitting semiconductor laser and to reduce the variation of the rise time according to the temperature change (see Japanese Patent No. 4,123,791 (JP4123791B)). This driving system drives the vertical-cavity-surface-emitting semiconductor laser by a voltage drive circuit at the rising, and then, corrects the rise time by changing the circuit to a current drive circuit. Each of the voltage drive circuit and the current drive circuit is provided with a correction circuit that corrects the variation of the light amount according to the temperature change.
However, since the driving system disclosed in JP4123791B requires both of the voltage drive circuit and the current drive circuit that have the correction circuits, there is a problem that the large circuit scale increases a cost.